122-28176 Parallax Inc, 122-28176 Datasheet - Page 232

• Manufacturer Part Number: 122-28176
• Description: GUIDE STUDENT PROCESS CONTROL
• Manufacturer: Parallax Inc

Specifications of 122-28176

• Accessory Type: Manual
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: Propeller Education (PE) Kit
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

desired output of the process. The controller evaluates the two pieces of data. Based on
this evaluation, the controller initiates action on the power interface. This block provides
the signal conditioning at the controller’s output. Chapter 5 discussed several methods of
driving power interface circuits. The power interface has the ability to control the
actuator. This may be a relay, a solenoid valve, a motor drive, etc. The action taken by
the actuator is sufficient to drive the actual output toward the desired value.
As you can see, this control scenario forms a loop, a closed-loop. Furthermore, since it is
the process’s output that is being measured, and its value determines actuator settings, it
is a feedback closed-loop system. The input changes the process output → the output is
monitored for evaluation → the evaluation changes the input → changes the process
output, etc., etc.
The type of reaction that takes place upon evaluation of the input defines the process-
control mode. There are five common control modes. They are on-off, on-off with
differential gap, proportional, integral, and derivative. The fundamental characteristic that
distinguishes each control mode is listed below in Table 7-1.
On-off
On-off with differential
gap
Proportional
Integral
Derivative
Process Control Mode
Table 7-1: Five Common Control Modes
Is the variable above or below
a specific desired value?
Is the variable above or below
a range defined by an upper
and lower limit?
How far is the measured
variable away from the desired
value?
Does the error still persist?
How fast is the error
occurring?
Evaluation
Drive the output fully ON or
fully OFF.
Output is turned fully ON and
fully OFF to drive the
measured value through a
range.
Take a degree of action
relative to the magnitude of
the error.
Continue taking more forceful
action for the duration the
error exists.
Take action based on the rate
at which the error is occurring.
Action